As small size electronic appliances have become portable in recent years, there has been an increase in demand for a lithium secondary battery having a small size, a light weight and a high energy density to replace nickel/cadmium batteries.
As active materials for a positive electrode in such a lithium secondary battery, LiCoO.sub.2 and LiNiO.sub.2 are layered compounds capable of being intercalated and deintercalated with lithium. Of them, LiNiO.sub.2 is being investigated because of its higher electric capacity than LiCoO.sub.2.
Usually LiNiO.sub.2 is prepared by mixing a lithium component (LiOH, Li.sub.2 CO.sub.3, LiNO.sub.3 etc.) with a nickel component (hydroxide, carbonate etc.) in a powdery form and reacting the mixture by the so-called dry process, and hence required heating at an elevated temperature for a long time. Especially in the case of Ni, it is hard to convert a divalent ion into a trivalent ion and therefore heating at an elevated temperature for a long time was essential. Consequently, as the crystal growth proceeds, but some of the lithium is evaporated off and NiO as a by-product is formed, thereby lowering the purity.
To the contrary, the present inventors succeeded in preparing highly purified LiNiO.sub.2 having a high crystallization degree by forming a uniform precursor of Li and Ni components using a wet process and heating it for a short time as disclosed in Japanese Patent Application No. 6-80895 (Japanese Patent Kokai NO. 8-130013).
As to LiNiO.sub.2, however, when much of the Li was evaporated off (at the time of discharging), the structure tended to become unstable owing to the two-dimensional structure. Therefore the essential problem of a poor cycle property of a lithium secondary battery could not be completely overcome. Although the effect of improving the cycle characteristics was achieved to some extent by using the technique of the Japanese Patent Application No. 6-80895, the improvement was still insufficient for long term cycle characteristics of more than 100 cycles.
Under such circumstance, many attempts have been made to stabilize the structure by substituting a part of the nickel with another component (third component). For example, active materials of positive electrode represented by Li.sub.y Ni.sub.x Co.sub.1-x O.sub.2 (wherein x is 0&lt;x.ltoreq.0.75 and y is y.ltoreq.1), where Co was doped in a solid solution into LiNiO.sub.2, and Li.sub.y Ni.sub.1-x Me.sub.x O.sub.2 (wherein Me represents any one of Ti, V, Mn and Fe, x is O&lt;x.ltoreq.0.6 and y is 0.2&lt;y.ltoreq.1.3), where Ti, V, Mn or Fe was doped as solid solution into LiNiO.sub.2 are disclosed in Japanese Patent Kokai Nos. 63-299056 and 5-283076 respectively.
However, the process by which the third component was doped in a solid solution was carried out by a dry process described above and hence it was difficult to homogeneously dope the third component in a solid solution. The process involved the increasing the amount of the third component, heating at an elevated temperature for a long time, and inevitably pulverizing several times. Consequently, Li was evaporated off and the by-product of NiO was formed, thereby lowering the purity and sufficient improvement in the cycle property could not be achieved, as with the LiNiO.sub.2 described above. Also, since the dry process required heating for a long time and pulverizing the product it was inefficient and uneconomical. Furthermore, since these dry processes take a long time for heating, it was impossible to adjust the crystal size to a desired level while keeping the crystallization degree and purity at high level.
Under these circumstances, attempts have been made to prepare spherical particles for increasing a packing density. For example, a technique is disclosed in Japanese Patent Kokai No. 7-105950 for preparing spherical LiNiO.sub.2 particles having 5 .mu.m.about.50 .mu.m using spherical Ni(OH).sub.2 as a raw material. This technique is used for obtaining spherical LiNiO.sub.2 by a dry process only for the purpose of increasing the packing density. But, the primary particle size and the purity of LiNiO.sub.2 was not taken into consideration. The result was not satisfactory. Another technique is disclosed in Japanese Patent Kokai No. 6-333562 for preparing spherical LiNiO.sub.2 having 0.1.about.1.1 .mu.m using a mist dry process. In this technique, the crystal size is too fine so that the crystals are passed through a separator when employed as a battery and thus it is not practical for battery use. Especially in case of LiNiO.sub.2, when the primary particles are too fine there is the problem that the storage stability is poor due to moisture absorption and so good and stable battery characteristics cannot be obtained.
Moreover, it is known that once a battery is exposed to an elevated temperature, for example in a car in the daytime, even if it is reverted to a normal temperature, the positive electrode active material is susceptible to great deterioration and discharge performance becomes poor so that the battery performance is greatly decreased.
As the means to prevent the deterioration of the positive electrode active material, in considering the fact that the finer the primary particle size, the greater the deterioration, the size of the primary particle of the active material to be prepared has been noticed.
As a process for improving the storage stability or the discharge characteristics of the positive electrode active material at high temperatures by making the primary particle size larger, there have been some attempts to make the primary particle size of the LiCoO.sub.2 series larger by improving the heating conditions [Japanese Patent Kokai No. 6-243897 (0.1.about.2. 0 .mu.m), Japanese Patent Kokai No. 6-325791 (0.01.about.5 .mu.m) and Japanese Patent Kokai No. 7-14579 (0.01.about.5 .mu.m) ].
Also, a primary particle having an average size larger than 2 .mu.m is prepared by adding Bi oxide to the raw material resource, as proposed in Japanese Patent Kokai No. 8-55624. Thus it is easy for the LiCoO.sub.2 series to have a larger primary particle size.
On the other hand, there has not yet been found an example in which the primary particle size of LiNiO.sub.2 is as large as in the LiCoO.sub.2, series. The reason is attributed to the fact that the synthesis of LiNiO.sub.2, or the third component (M) added to the related compound Li.sub.y Ni.sub.1-x M.sub.x O.sub.2, required heating at an elevated temperature for a long time because of their bad reactivity as described above, thereby Li is liable to be evaporated off. And consequently the crystal growth is difficult and the crystal becomes imperfect with many lattice defects. For this reason, the heating has been conducted at higher possible temperatures within the permissible temperature range and as a result only fine primary particles of less than 1 .mu.m could be obtained.